Phosphorous (P) is an essential element for growth. A substantial amount of the phosphorous found in conventional livestock feed, e.g., cereal grains, oil seed meal, and by-products that originate from seeds, is in the form of phosphate which is covalently bound in a molecule know as phytate (myoinositol hexakisphosphate). The bioavailability of phosphorus in this form is generally quite low for non-ruminants, such as poultry and swine, because they lack digestive enzymes for separating phosphorus from the phytate molecule.
Several important consequences of the inability of non-ruminants to utilize phytate may be noted. For example, expense is incurred when inorganic phosphorus (e.g., dicalcium phosphate, defluorinated phosphate) or animal products (e.g., meat and bone meal, fish meal) are added to meet the animals' nutritional requirements for phosphorus. Additionally, phytate can bind or chelate a number of minerals (e.g., calcium, zinc, iron, magnesium, copper) in the gastrointestinal tract, thereby rendering them unavailable for absorption. Furthermore, most of the phytate present in feed passes through the gastrointestinal tract, elevating the amount of phosphorous in the manure. This leads to an increased ecological phosphorous burden on the environment.
Ruminants, such as cattle, in contrast, readily utilize phytate thanks to an enzyme produced by rumen microorganisms known as phytase. Phytase catalyzes the hydrolysis of phytate to (1) myo-inositol and/or (2) mono-, di-, tri-, tetra- and/or penta-phosphates thereof and (3) inorganic phosphate. Two different types of phytases are known: (1) a so-called 3-phytase (myo-inositol hexaphosphate 3-phosphohydrolase, EC 3.1.3.8) and (2) a so-called 6-phytase (myo-inositol hexaphosphate 6-phosphohydrolase, EC 3.1.3.26). The 3-phytase preferentially hydrolyzes first the ester bond at the 3-position, whereas the 6-phytase preferentially hydrolyzes first the ester bond at the 6-position.
Microbial phytase, as a feed additive, has been found to improve the bioavailability of phytate phosphorous in typical non-ruminant diets (See, e.g., Cromwell, et al, 1993). The result is a decreased need to add inorganic phosphorous to animal feeds, as well as lower phosphorous levels in the excreted manure (See, e.g., Kornegay, et al, 1996).
Despite such advantages, few of the known phytases have gained widespread acceptance in the feed industry. The reasons for this vary from enzyme to enzyme. Typical concerns relate to high manufacture costs and/or poor stability/activity of the enzyme in the environment of the desired application (e.g., the pH/temperature encountered in the processing of feedstuffs, or in the digestive tracts of animals).
It is, thus, generally desirable to discover and develop novel enzymes having good stability and phytase activity for use in connection with animal feed, and to apply advancements in fermentation technology to the production of such enzymes in order to make them commercially viable. It is also desirable to ascertain nucleotide sequences which can be used to produce more efficient genetically engineered organisms capable of expressing such phytases in quantities suitable for industrial production. It is still further desirable to develop a phytase expression system via genetic engineering which will enable the purification and utilization of working quantities of relatively pure enzyme.